Magnetoresistive elements feature an electrical resistance that strongly depends on the magnitude and/or a direction of an externally applied magnetic field. Magnetoresistive elements therefore provide efficient determination of magnetic field strength and magnetic field direction. For example, when the resistance of an element varies with the angle between the element and the direction of an applied magnetic field, by making use of such an element, a rotation angle can be effectively measured in a touch less way.
In particular, by appropriately interconnecting four magnetoresistive elements, e.g. four identical spinvalves where two spinvalves are shielded in the form of a Wheatstone bridge, the direction of an externally applied magnetic field can be determined in an interval of 0° to 180°. Unequivocal determination of a direction of a magnetic field over a range from 0° to 360° can only be detected if spinvalves are used with different pinning directions.
Generally, there exists a large variety of magnetoresistive element types making use of different fundamental effects. For example, the Anisotropic Magnetoresistive (AMR) effect shows a change in electrical resistance in the presence of a magnetic field. AMR sensors are typically made of a soft-magnetic material, such as nickel-iron (Permalloy) thin film deposited on a silicon wafer. The magnetoresistive effect is mainly given by the relative direction between an electrical current and the direction of magnetization.
Another effect denoted as Giant Magneto Resistance (GMR) can be exploited by making use of multilayer systems. Here, the magnetoresistive element features a stack of alternating magnetic and non magnetic layers. Typically, the magnetic layers are ferromagnetic layers. The magnetizations of adjacent ferromagnetic layers are coupled in an anti-parallel orientation. The electrical resistance of such a GMR element strongly depends on the mutual orientation of the magnetization of adjacently positioned magnetic layers.
If a magnetic field is applied, the magnetic force between the magnetic field and the magnetization of the ferromagnetic layers attempts to align the magnetizations of the adjacent layers in a parallel way. Hence the orientation between two adjacent layers is something between parallel (very high field) and anti-parallel (very low field) and depends on the strength of the magnetic field. This results in a magnetic field-dependant resistance. The resistance has its maximum if the orientation between two adjacent layers is anti-parallel and the resistance has its minimum if the orientation between two adjacent layers is parallel. The non magnetic layer for separating two adjacent ferromagnetic layers may be either an electrically conducting or isolating type. In case of positioning an isolating material between the ferromagnetic layers of a GMR system the so called Tunnel Magneto Resistance (TMR) may exhibit.
Principally, GMR systems feature a larger change in electrical resistance compared to AMR systems. However, a GMR multilayer does not inherently allow measurement of a direction of an externally applied magnetic field. This can be achieved by coupling at least one magnetic layer, the pinned layer, to a magnetic layer with fixed spatial orientation, the so called pinning layer. The pinning layer in practice typically comprises an antiferromagnetic material. In this way the direction of magnetization of the pinned layer is spatially fixed while the other ferromagnetic layer features a magnetization that is free to follow an external magnetic field. Therefore, this layer is also denoted as “free layer”. These dedicated GMR systems featuring a pinned ferromagnetic layer are also denoted as spinvalves.
Sophisticated magnetic sensors may feature a plurality of GMR elements or spinvalves that have their magnetizations pinned in different directions. In principle, pinning of a magnetic layer can be effectively realized by means of an annealing process. Here, the entire magnetoresistive element is heated to a temperature above the antiferromagnetic layer's Neel temperature and an external magnetic field is applied in order to induce a uniform magnetization of the pinned layer. During subsequent cooling down the external magnetic field remains applied. After cooling down the magnetization of the pinned ferromagnetic layer is maintained by means of a magnetic coupling with the adjacently positioned anti-ferromagnetic bottom layer.
Manufacturing of GMR spinvalves is now subject to mass production. For example, several hundreds or thousands of GMR spinvalves can be produced on a single substrate or wafer. In these mass production processes, individual annealing of various particular GMR elements of a wafer of GMR elements is not possible, because an externally applied magnetic field for pinning of the ferromagnetic layers cannot be separately applied to distinct GMR elements of the wafer.
For many applications a plurality of identical GMR spinvalves with pinned layers of different orientation have to be assembled. Such an assembly can be in principle realized by mounting identical GMR spinvalves in different orientations. Such a process involves manual orientation and assembling of various GMR elements and is therefore time intensive and rather costly.
Another approach for creating an assembly of GMR spinvalves with different orientations may exploit various GMR elements featuring different annealing temperatures. Then, in two successive annealing processes making use of two different annealing temperatures, respective ferromagnetic layers can be separately oriented and fixed. However, this approach involves usage of different ferromagnetic materials that may also differ with respect to other physical properties, like electrical resistance and thermal properties.
Selective reorientation of ferromagnetic layers might also be realized by ion bombardment at the expense of an adverse impact on the electrical resistance of the GMR elements.
The present invention therefore aims to provide a time-and cost efficient way to produce a large amount of equivalent magnetoresistive elements with different magnetization directions.